1. Field of the Invention
The field of the present invention generally relates to the field of power converters, more particularly to medium voltage variable speed AC drives used to drive AC motors, and to drive various other power conversion equipment requiring 600 to 15,000 volts.
2. Discussion of Related Art
Adjustable or variable speed AC drives are commonly used to drive AC induction motors allowing for its speed control. AC drives generally provide the advantage of energy savings because they control the characteristics of its output voltage and current, and thus controlling the motor speed (of the motor they are driving) by the user, optimizing the motor power usage as well as the process it is driving. AC drives operate by taking either incoming AC or DC power, having a fixed frequency and voltage, and converting it to AC power having a voltage or current with variable amplitude and frequency. This allows for the control of the motor speed and power, a requirement in many applications.
Although the use of AC drives greatly enhances the processes they drive and provide electrical energy savings, their use is not without drawbacks. One of the biggest problems with current AC drive systems is their size. This is particularly true in AC drives used for medium voltage applications, which require a significant amount of floor or facility space. Additionally, because the AC drive components in existing systems are exposed to air (which is used as the means to cool the components of the systems, they are required to be located in clean, environmentally controlled areas requiring significant filtering, temperature and humidity control means, to keep harmful contaminants and humidity from adversely affecting the operation of the systems. Induction motors on the other hand are generally rugged not requiring a clean environment free of contaminants. This difference of environmental requirements will generally require a significant distance between drive and motor. This distance causes motor insulation problems due to reflective wave phenomena as well as motor dynamic control problems.
The state-of-the-art medium voltage power conversion devices use insulating methods similar to the ones used in low voltage (0 to 600V) devices. The power switches and interconnection buses have to have significant physical space between different electric potentials in order to avoid the effects of partial discharge degradation of the insulating materials. Because ambient air is present around all the components this distance can be fairly large. Moreover, the quality of the air incoming to the drive cabinet needs to be environmentally controlled because contaminants could surround the insulation devices and facilitate the ionization around the conductors and therefore generate corona effect or even cause a short-circuit. This large volume required for the sake of insulation will result in an increased stray inductance distributed in the interconnection devices. The interconnection devices can be either some sort of cables or bus bars. Because the power switches carry significant amount of electrical current, during the turnoff cycle the negative rate of change of the switch current will result in a voltage spike that is proportional to the magnitude of the current rate of change and the stray inductance of the circuit involved. (Vpk=L*(di/dt), where L is the circuit stray inductance and di/dt is the rate of change of the switch current.) These voltage transients will destroy the power semiconductor switches if not limited to adequate levels. State-of-the-art medium-voltage power conversion devices end up using voltage suppressing devices that are proportional to the amount of stray inductance. The addition of these transient voltage suppressors, which also require proper spacing due to insulation needs, ends up increasing the physical volume even further, as well as complexity and cost.
Another problem with many power conversion devices and in particular in AC drives is component replaceability. Because the internal make-up of present AC drives is generally complex, when one component, such as a semiconductor power switch fails, it is often the case that a significant amount of time is required to replace the damaged component.
A further problem of medium voltage AC drives is their susceptibility to cosmic rays. Although all of the aspects of this phenomenon are unknown, it is known that subatomic particles emitted from various celestial bodies impact the earth, and everything on it, including the components of power conversion systems. These high-energy particles cause the destruction of the semiconductor atomic structure and thus, its ability to withstand their rated voltage. Therefore, cosmic rays cause the deterioration over time of some of the electrically live components in AC drives, such as the semiconductor power switches like the IGBTxe2x80x94Insulated Gate Bipolar Transistor and the IGCTxe2x80x94Integrated Gate Commutated Thyristor. This phenomenon limits the voltage utilization of a power semiconductor switch to about sixty percent of its rated DC value. If used above this voltage level the power semiconductor switch will have a significant shorter life cycle.
The present invention is directed to solving the above problems in power conversion equipment, by allowing more compact devices which can be placed near, or directly adjacent to, the machine or load being driven, without any of the above adverse effects. This invention is a unique combination of available state-of-the-art and cutting edge technologies of different technological areas. It allows for an efficient, environmentally rugged, flexible and compact power conversion system for low, medium and high voltage applications. This invention enables the design of power conversion building blocks with unprecedented features using either gas or liquid fluid as coolants.
This invention can be applied to any power conversion device and in particular to medium voltage AC Drives. The electrical topology used in the present invention is similar to ones used in state of-the-art power conversion equipment. In the preferred embodiment, a multilevel AC drive topology is implemented. The plurality of inverters and converter are electrically coupled through electrical buses and physically coupled through their respective modular bases, and may share a common cooling system connected to the respective heat sinks of each component. The novel arrangement of the interconnecting bus and power components, the insulation, the cooling system and the cosmic ray filtering eliminate the problems stated above with present AC drives, while allowing the drive to have extended, efficient operational life, take a small amount of space, be easily repairable and be located in close proximity to the load device being driven. A more detailed discussion of the present invention will follow, starting with a brief description of the elements.
In the present invention, an AC drive is made up of a plurality of inverter modules, which are connected to a converter module to create the AC drive, wherein each of the above components is packaged in a relatively small unit having a cooling apparatus. Each of the inverters is made of a modular base, a heat sink or exchanger connected to the base having a plurality of power semiconductor switches thermally coupled thereto, a plurality of capacitors, a plurality of electrical buses connecting the power semiconductor switches to the capacitors, and an insulative medium which encases or covers some or all of the electrically live components, such as the electrical buses. The converter is similar in construction, in that it is made up of a modular base, a plurality of power semiconductor switches or diodes, capacitors. a reactor, a heat sink or exchanger, a plurality of contactors and an insulative material covering the electrically live components, such as the buses.